I have the below class (which is basically a wrapper for std::vector), and one of the functions returns a variable of the type AInteger (which is basically a wrapper for int). Now the type AInteger is used multiple times throughout the class, yet the compiler starts complaining at a very specific position. When I remove the "getSize()" function, everything compiles just fine.
There is mutual inclusion, so I need the forward declaration for everything to work.
One of the problems is that the AList class is a template, so it is impossible to move the definitions to a .cpp file (which would normally solve the problem).
What am I doing wrong?
Here's the class:
#ifndef ALIST_H
#define ALIST_H
#include <vector>
#include "AInteger.h"
class AInteger;
template<typename VALUE>
class AList {
public:
AList() {
}
AList(const std::vector<VALUE> list) {
value = list;
}
~AList() {
}
operator const std::vector<VALUE>() const {
return value;
}
std::vector<VALUE> toStdVector() const {
return value;
}
VALUE operator [](const AInteger index) const {
return value.at(index);
}
void add(const VALUE value) {
this->value.push_back(value);
}
VALUE get(const AInteger index) const {
return value[index];
}
AInteger getSize() const { // ERROR OCCURS HERE
return value.size();
}
void remove(const AInteger index) {
value.erase(index);
}
private:
std::vector<VALUE> value;
};
#endif
Output:
1>------ Build started: Project: ALibrary, Configuration: Debug Win32 ------
1> ASocket.cpp
1>d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(43): error C2027: use of undefined type 'AInteger'
1> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(8): note: see declaration of 'AInteger'
1> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(53): note: see reference to class template instantiation 'AList<VALUE>' being compiled
1> AInteger.cpp
1>d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(43): error C2027: use of undefined type 'AInteger'
1> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(8): note: see declaration of 'AInteger'
1> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(53): note: see reference to class template instantiation 'AList<VALUE>' being compiled
1> AHttpRequest.cpp
1>d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(43): error C2027: use of undefined type 'AInteger'
1> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(8): note: see declaration of 'AInteger'
1> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(53): note: see reference to class template instantiation 'AList<VALUE>' being compiled
1> ABoolean.cpp
1>d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(43): error C2027: use of undefined type 'AInteger'
1> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(8): note: see declaration of 'AInteger'
1> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(53): note: see reference to class template instantiation 'AList<VALUE>' being compiled
1> Generating Code...
1> Compiling...
1> AString.cpp
1> Generating Code...
2>------ Build started: Project: BitHoarder, Configuration: Debug Win32 ------
2> SystemHandler.cpp
2>d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(43): error C2027: use of undefined type 'AInteger'
2> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(8): note: see declaration of 'AInteger'
2> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(53): note: see reference to class template instantiation 'AList<VALUE>' being compiled
2> Main.cpp
2>d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(43): error C2027: use of undefined type 'AInteger'
2> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(8): note: see declaration of 'AInteger'
2> d:\programs\programming\visual studio projects\c++\alibrary\alibrary\alist.h(53): note: see reference to class template instantiation 'AList<VALUE>' being compiled
2> Generating Code...
========== Build: 0 succeeded, 2 failed, 0 up-to-date, 0 skipped ==========
If any more code is needed just ask and I will provide.
Thanks!
EDIT:
Here's the code that's inside "AInteger.h"
#ifndef AINTEGER_H
#define AINTEGER_H
#include "AList.h"
template<typename VALUE>
class AList;
class AInteger {
public:
AInteger();
AInteger(const int);
~AInteger();
operator const int() const;
int toInt() const;
AInteger operator=(const AInteger &);
AInteger & operator++();
AInteger & operator--();
AList<AInteger>splitByNumber(const AInteger &);
private:
int value;
};
#endif
To make matters even more confusing, the following class, which does THE EXACT SAME THING does not produce the error:
#ifndef ADICTIONARY_H
#define ADICTIONARY_H
#include <map>
#include "AInteger.h"
class AInteger;
template<typename KEY, typename VALUE, typename COMPARE = std::less<KEY>>
class ADictionary {
public:
ADictionary() {
}
ADictionary(const std::map<KEY, VALUE, COMPARE> dictionary) {
value = dictionary;
}
~ADictionary() {
}
operator const std::map<KEY, VALUE, COMPARE>() const {
return value;
}
std::map<KEY, VALUE, COMPARE> toStdMap() const {
return value;
}
VALUE operator [](const KEY key) const {
return value.at(key);
}
void add(const KEY key, const VALUE value) {
this->value.insert(std::make_pair(key, value));
}
VALUE get(const KEY key) const {
return value[key];
}
AInteger getSize() const { // No error here
return value.size();
}
void remove(const KEY key) {
value.erase(value.find(key));
}
private:
std::map<KEY, VALUE, COMPARE> value;
};
#endif
The implementation of getSize() must create an instance of AInteger. This is only possible if the full definition of AInteger is known. You cannot create an instance of a type which is only forward-declared.
And that's exactly what the compiler tells you: error C2027: use of undefined type 'AInteger'. It does not ignore the forward declaration but tells you that it's not enough.
As for the #include "AInteger.h", you do not show its contents, but possible problems are:
Buggy include guard in the header file causes the compiler to skip the definition.
The header file defines a different type with a similar name.
The definition of AInteger is within a namespace.
What's gone wrong is answered very well in Christian Hackl's answer and normally I'd drop it there, but I can't explain how the OP can fix this properly in a comment.
First a revised AInteger.h
#ifndef AINTEGER_H
#define AINTEGER_H
template<typename VALUE>
class AList;
class AInteger {
public:
AInteger();
AInteger(const int);
~AInteger();
//operator const int() const;
int toInt() const;
AInteger operator=(const AInteger &);
AInteger & operator++();
AInteger & operator--();
std::unique_ptr<AList<AInteger>> splitByNumber(const AInteger &);
void splitByNumber(const AInteger &,
AList<AInteger> &);
private:
int value;
};
#endif
The include of AList.h is gone. The forward declaration of AList remains I've provided two different splitByNumber methods. Pick one. The first creates and returns a pointer protected by a smart pointer. The second approach, and my personal preference, takes a reference to an AList created by the caller.
The thing is neither know anything about the inner workings of AList because all they care is about is A) It exists and b) they can get the address of one.
Bear with me while I explore both because I think they are both educational.
Alist.h remains unchanged except for the removal of the forward declaration of AInteger.
The two splitByNumber candidates sit in AInteger's implementation file which can safely include both AInteger.h and AList.h and thus has complete knowledge of both at the same time and can do all the magical things AIntegers and ALists can do.
std::unique_ptr<AList<AInteger>> AInteger::splitByNumber(const AInteger & integer)
{
std::unique_ptr<AList<AInteger>> listp(new AList<AInteger>());
// do stuff with listp
return listp;
}
void AInteger::splitByNumber(const AInteger & integer,
AList<AInteger> & list)
{
// do stuff with list
}
Using the pointer version:
std::unique_ptr<AList<AInteger>> alistp = aitest1.splitByNumber(aitest2);
alistp->add(somenumber);
Using the reference version:
AList<AInteger> alist; // first create an empty AList
aitest1.splitByNumber(aitest2, alist); // pass it into the function
alist.add(somenumber); // use the AList
Related
While creating a subclass of a template class I noticed I got an error on an overloaded function.
This compiler error was correct since one of the overloads was using a copy and the type was not copyable.
However, I was not using that function (correct overload or not). So I was surprised to get this error
After searching around a bit and reproducing in godbolt the culprit seemed to be __declspec(dllexport).
Reproduction in godbolt
Removing the declspec seems to result in the correct compilation.
Code in godbolt:
#include <memory>
#include <vector>
using namespace std;
template<class V>
struct Foo{
void update(const V& v);
void update(V&& v);
std::vector<V> values;
};
template<class V>
void Foo<V>::update(const V& v)
{
values[0] = v;
}
template<class V>
void Foo<V>::update(V&& v)
{
values[0] = std::move(v);
}
struct __declspec(dllexport) Bar : public Foo<std::unique_ptr<int>>
{
};
int main()
{
Bar f;
auto i = std::make_unique<int>(5);
//f.update(i);
//f.update(std::move(i));
}
My questions are mostly, how is declspec causing this behavior?
And, is there anything that can be done about this in the template class or derived class?
Error log:
(19): error C2280:
'std::unique_ptr>
&std::unique_ptr>::operator =(const
std::unique_ptr> &)': attempting to
reference a deleted function with [
_Ty=int ] C:/data/msvc/14.16.27023.1/include\memory(2338): note: see declaration of
'std::unique_ptr>::operator =' with
[
_Ty=int ] C:/data/msvc/14.16.27023.1/include\memory(2338): note: 'std::unique_ptr>
&std::unique_ptr>::operator =(const
std::unique_ptr> &)': function was
explicitly deleted with [
_Ty=int ] (18): note: while compiling class template member function 'void
Foo>>::update(const V &)'
with [
_Ty=int,
V=std::unique_ptr> ] (29): note: see reference to class template instantiation
'Foo>>' being compiled
with [
_Ty=int ]
Why is VS giving me this error? I am quite new to OOP and Templates in C++ and I am trying to write simple Stack implementation using these techniques. I tried to google it but I did not find anything useful.
Code:
#ifndef _STACK
#define _STACK
template <class T>
class Stack {
public:
Stack();
Stack(const int &num);
Stack(const Stack<T> &obj);
Stack& operator=(const Stack<T> &obj);
~Stack();
private:
T * buffer;
unsigned int actualSize;
unsigned int maxSize;
};
template <class T>
Stack<T>::Stack() : maxSize(5), actualSize(0), buffer(new T[5]) {};
template <class T>
Stack<T>::Stack(const int &num) : maxSize(num), actualSize(num), buffer(new T[num]) {};
template <class T>
Stack<T>::Stack(const Stack<T> &obj) {
this->actualSize = obj.actualSize;
this->maxSize = obj.maxSize;
std::copy(obj.buffer, obj.buffer + obj.actualSize, this->buffer);
}
template <class T>
Stack<T>& Stack<T>::operator=(const Stack<T> &obj) {
T * temp = new T[obj.actualSize];
std::copy(obj.buffer, obj.buffer + obj.actualSize, temp);
this->actualSize = obj.actualSize;
this->maxSize = obj.maxSize;
delete[] buffer;
buffer = temp;
return *this;
}
template <class T>
Stack<T>::~Stack() {
delete[] buffer;
}
#endif
Full build log - obviously there is a problem with std::copy :
1>------ Build started: Project: Stack, Configuration: Debug Win32 ------
1>main.cpp
1>c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\xutility(2483): warning C4996: 'std::copy::_Unchecked_iterators::_Deprecate': Call to 'std::copy' with parameters that may be unsafe - this call relies on the caller to check that the passed values are correct. To disable this warning, use -D_SCL_SECURE_NO_WARNINGS. See documentation on how to use Visual C++ 'Checked Iterators'
1>c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.14.26428\include\xutility(2483): note: see declaration of 'std::copy::_Unchecked_iterators::_Deprecate'
1>c:\users\peter\documents\pb071hw\circle\stack\stack.h(34): note: see reference to function template instantiation '_OutIt std::copy<T*,T*>(_InIt,_InIt,_OutIt)' being compiled
1> with
1> [
1> _OutIt=int *,
1> T=int,
1> _InIt=int *
1> ]
1>c:\users\peter\documents\pb071hw\circle\stack\stack.h(32): note: while compiling class template member function 'Stack<int> &Stack<int>::operator =(const Stack<int> &)'
1>c:\users\peter\documents\pb071hw\circle\stack\main.cpp(13): note: see reference to function template instantiation 'Stack<int> &Stack<int>::operator =(const Stack<int> &)' being compiled
1>c:\users\peter\documents\pb071hw\circle\stack\main.cpp(10): note: see reference to class template instantiation 'Stack<int>' being compiled
1>Stack.vcxproj ->
I'm trying to make a for_each function for a generic object that uses an size function and an item index function. But I'm having some difficulty with the syntax.
This is what I have so far (starting at line 128):
class base1
{
protected:
std::vector<int> items;
public:
base1()
: items({1,2,3})
{
}
int GetCount() const
{
}
};
class base2 : public base1
{
public:
base2()
: base1()
{
}
int GetItem(int i) const
{
return items[i];
}
};
class derived : public base2
{
public:
derived()
: base2()
{
}
};
template <typename CONTAINER, typename CONTAINER_BASE1, typename CONTAINER_BASE2, typename SIZE, typename CONTAINED, typename FUNC>
void for_each(CONTAINER* container, SIZE (CONTAINER_BASE1::*GetSize)() const, CONTAINED (CONTAINER_BASE2::*GetItem)(SIZE) const, FUNC& body)
{
for (SIZE i = 0; i < container->*GetSize(); ++i)
{
body(container->*GetItem(i));
}
}
void fn()
{
derived x;
for_each(&x, &derived::GetCount, &derived::GetItem, [](int i){
++i;
});
}
Right now, I get an error from VC++ 2013 stating:
1>d:\projects\test\test.cpp(169): error C2064: term does not evaluate to a function taking 0 arguments
1> d:\projects\test\test.cpp(180) : see reference to function template instantiation 'void for_each<derived,base1,base2,int,int,fn::<lambda_862ea397905775f7e094cde6fe9b462c>>(CONTAINER *,SIZE (__thiscall base1::* )(void) const,CONTAINED (__thiscall base2::* )(SIZE) const,FUNC &)' being compiled
1> with
1> [
1> CONTAINER=derived
1> , SIZE=int
1> , CONTAINED=int
1> , FUNC=fn::<lambda_862ea397905775f7e094cde6fe9b462c>
1> ]
1>d:\projects\test\test.cpp(171): error C2064: term does not evaluate to a function taking 1 arguments
Any ideas as to what the problem is?
You have two bugs. You take the functor by non-const lvalue reference - FUNC& body - which doesn't bind to a temporary like a lambda; this was hidden by a terrible MSVC extension that allows such bindings. You should accept the function object by value (the way it is usually done by the standard library), by const lvalue reference (if copying is expensive and/or identity is important), or by forwarding reference (if identity is important and operator() can be non-const).
Second is operator precedence. The postfix function call operator has higher precedence than .* and ->*. container->*GetSize() is container->*(GetSize()); you want (container->*GetSize)().
I'm also not sure about this design. It's probably better to provide a uniform interface, and simply do, e.g., container.size() and container.at(i) than using this tortured system of pointer-to-member-functions.
Now I'm migrating my project from Visual Studio 2008 to 2013 (with no updates installed), and facing a problem.
I have a sort of Variant type, CData< T>, that has a conversion operators to the contained type T.
template<class T> T& GetValue();
template<class T> const T& GetValue() const;
template<class T> T* GetValuePtr();
template<class T> const T* GetValuePtr() const;
template<class T> operator T&() { return GetValue<T>(); }
template<class T> operator const T&() const { return GetValue<T>(); }
template<class T> operator T*() { return GetValuePtr<T>(); }
template<class T> operator const T*() const { return GetValuePtr<T>(); }
There are methods in a class CDataArray:
CData& Get(int Index) { return At(Index); }
const CData& Get(int Index) const { return operator [](Index); }
template<class T>
T& Get(int Index) { return At(Index).GetValue<T>(); }
template<class T>
const T& Get(int Index) const { return operator [](Index).GetValue<T>(); }
and there is a call to it:
Data::PDataArray SGQuests;
Data::PParams SGQuest = SGQuests->Get(i);
Here, Data::PParams is going to be T.
In VS2008, it seems it used non-const non-template CDataArray::Get(), which returned CData&, and then this CData& was accessed with non-const template operator T&(), finally returning Data::PParams&.
In VS2013, it for some reason uses a const overload and it results in error:
1>PATH_TO_SRC\l1\data\type.h(126): error C2678: binary "=": no operator found that accepts left operand "const Ptr<Data::CParams>" (or there is no acceptable conversion)
1> PATH_TO_SRC\l1\data\ptr.h(29): may be "void Ptr<Data::CParams>::operator =(T *)"
1> with
1> [
1> T=Data::CParams
1> ]
1> PATH_TO_SRC\l1\data\ptr.h(28): or "void Ptr<Data::CParams>::operator =(const Ptr<Data::CParams> &)"
1> trying to match argument list "(const Ptr<Data::CParams>, const Ptr<Data::CParams>)"
1> PATH_TO_SRC\l1\data\type.h(125): compiling member function "void Data::CTypeImpl<T>::Copy(void **,void *const *) const" template class
1> with
1> [
1> T=const Ptr<Data::CParams>
1> ]
1> PATH_TO_SRC\l1\data\data.h(167): "Data::CTypeImpl<T>"
1> with
1> [
1> T=const Ptr<Data::CParams>
1> ]
1> PATH_TO_SRC\l1\data\data.h(97): "T &Data::CData::GetValue<T>(void)"
1> with
1> [
1> T=const Ptr<Data::CParams>
1> ]
1> PATH_TO_SRC\l3\quests\questmanager.cpp(307): "Data::CData::operator const T(void)<const Ptr<Data::CParams>>"
1> with
1> [
1> T=const Ptr<Data::CParams>
1> ]
I translated messages to English manually, so they can slightly differ from original EN compiler messages.
And finally, if I write explicit template argument:
Data::PParams SGQuest = SGQuests->Get<Data::PParams>(i);
It compiles OK.
The questions are:
How VS2013 template argument guessing differs from VS2008 one? Is there any place (standard, article or smth) that clearly explains, why my old code shouldn't compile? Any reference would be appreciated.
How should I write code in that situations? Do I have to write template arguments explicitly now, or just modify member overloads, or install some update?
P.S. Minimal code is available at https://www.dropbox.com/s/zjohnu5v87tyr2c/ConstOverload.zip?dl=0 . Full source code is at
https://code.google.com/p/deusexmachina/source/browse/branches/Dev/DEM/Src
This problem was solved by adding third overload to CData
template<class T> operator const T&() { return GetValue<T>(); }
Original solution is posted in:
Conversion operator error C2678 in VS2013, works in VS2008
Look there for possible further details.
I've heard a little bit about reference-to-reference problem and this resolution. I'm not very good with C++ Committee terminology, but I understand the "Moved to DR" annotation in the link means that this is the current interpretation that standard-conforming compilers should adhere to.
I have this sample code that I can't understand:
template <typename T>
struct C {
void f(T&) { }
void f(const T&) { }
};
int main() {
C<int> x; // OK
C<int&> y; // compile error: f cannot be overloaded
C<const int&> z; // compile error: f cannot be overloaded
}
I understand the error in C<const int&> case: using rules from DR #106 we get two methods with the same signature f(const int&). What I don't get is the C<int&> case: shouldn't it generate exactly the same code as C<int> (at least according to Stroustrup's resolution)?
DR only means "Defect Report", and to my knowledge, the described resolution hasn't made it (yet) to the standard. For this reason, I believe a strictly conforming C++03 implementation should not compile this code because of it is forming a reference to a reference.
[Edit] Just found a nice answer on this issue.
Interestingly, when I compile your code (Visual C++ 10 Express) I get errors, but also when I try this simpler case:
int main(int argc, char* argv[])
{
C<int> x; // OK
C<const int> x1; // error C2535: 'void C<T>::f(T &)' : member function
// already defined or declared
return 0;
}
Seems like the ref-to-ref collapsing defined in the DR you mentioned means that const ref becomes a simple non-const ref within the template. My problem with this is that I don't understand why the second f is not just ignored.
If I change C so that the second f is const-qualified, this now compiles:
template <typename T>
struct C {
void f(T&) { }
void f(const T& t) const {}
};
The implication seems to be that when C is instantiated with const anything (ref or not), the two C::f overloads are simply identical, and result in compile-time duplicate detection.
Perhaps somebody smarter than me can decipher the chain more definitively here.
EDIT: On reflection, it's not surprising here that T = const int& results in the f overloads being identically instantiated as
void f(const int&) {}
That's what the compiler is telling me:
#include "stdafx.h"
template <typename T>
struct C {
void f(T&) { }
void f(const T&) { }
};
int main() {
C<const int&> z; // compile error: f cannot be overloaded
return 0;
}
gives this error:
1>test.cpp(6): error C2535: 'void C<T>::f(T)' : member function already
defined or declared
1> with
1> [
1> T=const int &
1> ]
1> test.cpp(5) : see declaration of 'C<T>::f'
1> with
1> [
1> T=const int &
1> ]
1> test.cpp(10) : see reference to class template instantiation
'C<T>' being compiled
1> with
1> [
1> T=const int &
1> ]
I'm not even convinced this has anything to do with the DR.